Hydrocarbon and other resources that have been discovered can generally be made accessible by drilling wells down to the formation. In order to maintain the integrity of the well and to prevent cave-ins of the earth material, a rigid casing is installed in the wellbore. The underground deposits of the resources may occur at various levels in the wellbore. Completion of the well is generally accomplished by lowering one or more perforating guns into the wellbore casing, detonating the perforating gun(s) to perforate the casing sidewall at the various locations where the deposits exist. This allows the hydrocarbon or other resources to flow into the casing and be removed by pumps or other lift equipment.
In view that many hydrocarbon deposits exist in naturally-pressurized areas, the drilling, casing and perforating must be carried out with respect to the underground pressurized deposits. Drilling equipment and drill muds are often used to contain the well pressure once the wellbore has been drilled down to the formation. Capped well casings and other containment equipment are utilized during the casing and perforation operations to complete the well. It can be appreciated that in order to provide safe conditions and prevent waste of the resources, it is often necessary to conduct the well operations while maintaining confinement of the wellbore. To that end, it is necessary to minimize the number of times equipment is placed in or removed from a contained or capped wellbore.
The perforating operation is one activity that often requires the cased wellbore to be accessed several times. For example, the perforating gun must be lowered into the cased wellbore by a tubing string, wire line, or other equipment. Once the gun is detonated and the sidewall of the casing is perforated, the wellbore becomes pressurized as a result of high pressures in the formation forcing the hydrocarbons to flow through the perforations into the wellbore. At that time, it becomes extremely difficult to remove the perforating gun from the pressurized wellbore. In addition, the initial in rush of the hydrocarbons into the wellbore can often jam the suspended perforating gun and other equipment within the casing, thereby necessitating the removal thereof under the pressurized conditions.
In order to obviate the foregoing problem, perforating guns and associated equipment have been devised by those skilled in the art to eliminate the necessity of removing the spent perforating gun from the cased wellbore. U.S. Pat. Nos. 4,776,393 by Forehand et al.,; 5,050,682 by Huber et al.; and 5,156,213 by George et al. disclose various types of perforating guns and associated equipment. In general, the equipment in these noted patents allows the perforating gun to be lowered to the particular location in the cased wellbore and attached to the sidewalls thereof. Then, the tubing string, wire line or other cables can be removed from the well, and the well can be capped or otherwise contained. The perforating gun can be detonated by mechanical, hydraulic, or electrical techniques to thereby form the perforations in the sidewall of the casing. In response to the detonation of the perforating gun, a release mechanism is activated to release the equipment from the casing, thereby allowing it and the perforating gun to fall to the bottom of the well. In this manner, concerns in removing the perforating apparatus in a pressurized wellbore are thereby reduced. However, the perforating gun equipment disclosed in the above patents is subject to breakage and failure, thus representing a less than optimum solution. In addition, should the equipment, and especially the perforating gun, fail to operate, it must then be disengaged from the casing and removed so it can be repaired or replaced by other equipment.
From the foregoing, it can be seen that a need exists for a new, highly efficient, perforating and associated apparatus for providing a reliable and high quality attachment to the internal sidewall of the casing, as well as reliable operation in the disengagement thereof. Another need exists for a technique that is highly reliable in setting the equipment to the sidewall of the cased wellbore. Yet another need exists for a method and apparatus for retrieving the perforating gun and apparatus should it fail to fire or operate properly.